As is known, in various sectors of microelectronics there is an ever increasing need to integrate, in a single chip, various devices, which are specifically designed and optimized for a particular application, so as to obtain efficient and extremely compact dedicated systems (System-on-Chip). By way of example, it may be recalled that in important fields of use, such as that of mobile telephones, microprocessor cards (smart cards), image-acquisition devices (digital videocameras and cameras), palm-top computers, in which fundamental objectives are the reduction of overall dimensions, the reduction of supply voltages, and the reduction of consumption in general.
To achieve this purpose, it is particularly important to succeed in integrating logic devices for processing and control, generally referred to as digital-signal processors (DSPs), and memory devices (the so-called “embedded memories”). Furthermore, an individual integrated system must normally comprise both nonvolatile memories and volatile memories. The nonvolatile memories, which are usually of the EEPROM or flash type, are in fact necessary for permanent storage, on the one hand, of portions of program code that are to be executed by the processing and control devices, and on the other, data acquired during operation, such as, for example, digital images. The volatile memories, for example of the DRAM type, are instead used as work memories, on account of their performance in reading and writing, which is considerably higher than the performance of nonvolatile memories.
The integration, in a single device, of logic, of volatile memories, and of nonvolatile memories involves, however, problems that are chiefly due to the fact that the memory cells of the two types have different structures and must therefore be made with different technologies. For each type of cell, it is thus necessary to envisage specific process steps which generally cannot be executed simultaneously, but only in succession. Consequently, the processes of fabrication of a device that incorporates logic and both types of memory are complex and have a high cost.